Both Oklahoma's U.S. Senators, Jim Inhofe and James Lankford joined with 45 other Republican senators on Monday and signed an open letter to the leaders of Iran.
In the letter, Republican lawmakers warned that any nuclear deal Iran cuts with President Obama could expire the day he walks out of the Oval Office.
Republican senators worry that Iran is not negotiating in good faith and that a deal would be insufficient and unenforceable, allowing Iran to eventually become a nuclear-armed state.
The letter comes just days after Israeli Prime Minister Benjamin Netanyahu spoke to a joint meeting of Congress at Republican House Speaker John Boehner's invitation. In his address, Netanyahu bluntly warned the United States that a deal would pave Iran's path to a nuclear bomb.
The letter, written by Senator Tom Cotton of Arkansas, was addressed to the "Leaders of the Islamic Republic of Iran" and presents itself as a constitutional primer to the government of an American adversary.
Explaining the difference between a Senate-ratified treaty and a mere agreement between Obama and Iran's Ayatollah Ali Khamenei, the senators warned, "The next president could revoke such an executive agreement with the stroke of a pen, and future Congresses could modify the terms of the agreement at any time."
Cotton defended the letter in a series of television appearances Tuesday, denying emphatically that it undermines Obama's negotiating position with Iran.
Iranian Foreign Minister Mohammed Javad Zarif was quoted by the website of Iranian state TV on Tuesday as saying the letter's warning that any nuclear deal could be scrapped once Obama leaves office suggests the United States is "not trustworthy." He called the letter "unprecedented and undiplomatic." Earlier, he had dismissed it as a "propaganda ploy."
Negotiating alongside the U.S. are Britain, China, France, Germany and Russia. Nuclear negotiations resume next week in Switzerland.
The Associated Press contributed to this report.